The principle of an existing pilot-operated solenoid valve is as follows. When power is on, a pilot hole is opened by electromagnetic force and the pressure of an upper chamber drops rapidly. Thus, a pressure difference is formed around a closing piece due to a higher pressure at the top and a lower pressure at the bottom. The closing piece is pushed to move upward by fluid pressure and a valve is opened. When power is off, the pilot hole is closed by spring force and an inlet pressure enters into a chamber rapidly through a by-pass hole. Thus, a pressure difference is formed around the valve closing piece due to a higher pressure at the top and a lower pressure at the bottom. The closing piece is pushed to move downward by fluid pressure and the valve is closed. Generally, a ferromagnetic piston core is installed at an upper end of a fluid outlet of the valve. The lower end of the piston core is provided with a rubber ring for sealing, and the upper portion thereof is inserted into a valve cage made from non-magnetic material and presses against a pressure spring within the valve cage. When current is not supplied to a coil, due to an elastic restoring force of the pressure spring within the valve cage, the rubber ring at the lower end of the piston core is tightly pressed on a fluid outlet of the valve, therefore the valve is in a state of being closed off. When the coil is powered to operate, the resulting magnetic force urges the piston core to move upward and thus the rubber ring moves away from the fluid outlet within the valve, the valve is opened. When the coil is powered off, the piston core again closes off the valve under the action of the pressure spring. Drawbacks of this kind of electromagnetic valve include that the coil consumes a large amount of electric energy and causes serious heating and even package burning when it works. Additionally, in this kind of electromagnetic valve, the opening and closing of the valve adopts an instantaneous mechanism, causing interference in the fluid.